All play up their low fares, as do the U.S. discounters, which also play up the little things that set them apart: JetBlue and Virgin America boast frills like seatback screens, while defiantly Spartan Southwest offers two free checked-bags.

But when you get right down to it, for the passengers, it's all about the price of the ticket. And when it comes to the cheapest of the cheap, the most flagrantly frugal, the paradigm of parsimony -- two names stand apart: Ireland's Ryanair and the USA's Spirit.

Bottom line: Spirit and Ryanair often have the cheapest airfares. But sometimes, they don't. If you fail to compare prices, you might miss out on a better deal on another airline, and that includes the non-discounters.

Plus, consumers have to pay attention to the details of any airfare offering. Like Spirit's $9 specials. Now, a $9 flight is beyond cheap, but sadly, such bargains cannot be had on Spirit (or any other airline). Here's what you may not know about these airfares:

$9 is the one-way price.

$9 fares are only available to $9 Fare Club members (annual dues: $59.95).

$9 fares do not include all taxes and fees.

Your $9 airfare could wind up costing you close to $100 or even much more because that last bullet is the important one: Forget the additional government fees. The big add-ons come from the carrier, and Spirit makes no bones about it, though it often employs sprightly marketing terms like "empowerment" to describe its fees.

As Spirit spokesman Misty Pinson said, "[Customers] only pay for what they use," which is why Spirit calls them "optional fees." Let's look at two of the bigger optional fees:

Spirit bag fees. There are no free bags on Spirit. In fact, your little carry-on can cost you more ($20 to $40) than checking a giant Samsonite ($18 to $43). Your option? Stuffing your pockets with clean underwear and a fresh shirt, I guess, or buy new clothes on arrival. Or wear what you wore on the plane.

Spirit Passenger usage fee. This newly hiked fee of $16.99 each-way "applies to most reservations." Translation: You pay nearly $34 for the privilege of booking a round-trip flight online. Your alternative is to drive out to the airport and book there for free. Not much of an option.

But is there anything wrong with all these fees? Not at all. As long as you understand them. And while I had no trouble finding any of these fees on Spirit's website, the Department of Transportation slapped the Florida-based carrier with a $50,000 fine earlier this week for a couple of violations of its full fare disclosure rules in Spirit ads last June. The airline said it was "accidental" and will get the fine halved if no further violations occur within a year.

Again, know what you're getting for your money. Spirit has many fierce fans who do know what they get and prove it by flying the airline again and again. Newcomers, however, must do a little homework so they understand all those "optional" fees.

You could say the same for Dublin-based Ryanair, another cheapie with similar fee options. But don't look for market-speak from Ryanair CEO Michael O'Leary. After recently raising eyebrows by suggesting he would charge a fee for allowing passengers to watch inflight porn, O'Leary told the Guardian, "The gobbier I get, the more cheap headlines I get, the more tickets we sell."

This is the same man who wanted to install pay toilets on planes: "I still want to do that," he said, "but it rests on a misperception to suggest we'd make money from charging for using toilets. We'd give that money to an incontinence charity."

The real profit would come from removing some of the lavatories to add more seats.

That's one of the secrets of low-cost carriers: cramming more people into planes than other airlines do.

Spirit put a different spin on the concept last year when it boasted about some of its brand new seats: "They're pre-reclined!" What the carrier failed to note was these seats do not recline at all! Passengers may not be thrilled, but Spirit loves them because they take up less space, which means more passengers on the planes and more revenue for Spirit's bottom line.

And that's what it's all about for every airline. As long as you know what you'll pay in total, and what you'll get for that money, it's all good. And yes, sometimes Spirit Airlines is cheapest, and sometimes Ryanair is -- or Southwest or JetBlue -- but sometimes the legacy carriers win the cheapest honors, and that's because all airlines subscribe to the same motto: Never be a dollar more or less than the competition.

If airline charge less, they're not squeezing as many bucks out of us as they can; if they charge more, they won't end up on page one of your airfare search site results, and you won't fly them. By all means, fly your favorite airline whether it's Spirit or Ryanair or Wizz Air or United, but always compare prices before you buy.

Remember, just because an airline has a zany name like Cheapskate Air doesn't mean it'll always have the lowest fares.